A magnetorheological fluid (MRF) is a rapidly developing smart material. When the MRF is subjected to a magnetic field, solid magnetic particles within the MRF are caused to align themselves with the direction of the magnetic lines of force between the N pole and the S pole and accordingly form a plurality of particle chains between the two poles. The formation of the particle chains in the magnetorheological fluid produces an effect of anti-shear stress.
Among the conventional resistance devices that employ the MRF, there is a type of internal rotary resistance device, which mainly includes an outer cylinder, an inner magnetizable body, and a magnetorheological fluid. The outer cylinder encloses the inner magnetizable body therein. A plurality of magnetizable bars is circumferentially arranged on the inner magnetizable body, and each of the magnetizable bars has a coil wound thereon to serve as an applied magnetic field. The magnetizable bars with the coils wound thereon respectively have two ends outward extended through the outer cylinder. When the coils are supplied with a voltage or an electric current, a magnetic field is generated. The inner magnetizable body is rotatable relative to the outer cylinder. And, the magnetorheological fluid is filled in a space formed between the outer cylinder and the inner magnetizable body.
The manner in which the conventional internal rotary resistance device provides the resistance is described below. When the inner magnetizable body rotates about and relative to the outer cylinder, a voltage or an electric current can be supplied to the coils to generate a magnetic field. At this point, magnetic particles in the magnetorheological fluid located within the acting area of the magnetic field would align with the direction of the magnetic lines of force extended between the North and the South pole of the magnetic field to form particle chains between the two poles, which in turn produces an effect of anti-shear stress on an outer surface of the inner magnetizable body and an inner surface of the outer cylinder, preventing the inner magnetizable body from rotating relative to the outer cylinder, so as to achieve the purpose of providing a resistance.
With the structural arrangements of the conventional internal rotary resistance device, an increased resistance can be provided only when the space formed between the outer cylinder and the inner magnetizable body is increased to allow for more contact areas with the magnetorheological fluid. In this case, the internal rotary resistance device would have a disadvantageously expanded volume.